This can happen in the case of some BLOB columns. Binary Large Objects get scattered around the transaction log and sometimes can't be recovered for that reason. If this is not a text/ntext/image column please let me know.

The data from the log could be unavailable because the data has been overwritten in the log. This could happen on databases that use the Full recovery model when a full backup is done. It could also be that the database is using the simple recovery model. In the latter case, there is no guarantee that the data will be available.